The present invention relates to a self-oscillation type semiconductor laser used as a light source for an optical disk system and an optical disk device using such a semiconductor laser.
With the recent increase in demand for semiconductor lasers in the fields of optical communications, laser printers, optical disk devices, and the like, semiconductor lasers of the GaAs type and the InP type, mainly, have been actively studied and developed. In the optical information processing field, a method of recording and reproducing information using light from an AlGaAs type semiconductor laser with a wavelength of 780 nm, especially, has been commercialized. Such a method has been widely used for compact disks and the like.
In recent years, optical disk devices with a larger memory capacity have been increasingly in demand. With this demand, shorter-wavelength lasers have been requested. An AlGaInP type semiconductor laser can oscillate in the red region of wavelengths of 630 to 690 nm, emitting light with the shortest wavelength among those obtained from semiconductor lasers practically available at present. This type of semiconductor laser is therefore highly expected to be a next-generation large-capacity light source for optical information recording, replacing the conventional AlGaAs type semiconductor laser. In general, when reproducing information from an optical disk, a semiconductor laser generates intensity noise due to return of light reflected from a disk surface and temperature change, inducing a signal read error. A laser with low intensity noise is therefore indispensable for a light source of an optical disk.
Conventionally, in order to reduce noise, a low-output AlGaAs type semiconductor laser for a reproduction-only device has a structure where saturable absorbers are intentionally formed on each side of a ridge stripe. With this structure, multiple longitudinal modes can be obtained. In the case where disturbances such as return light and temperature change arise when a laser is oscillated in a single longitudinal mode, oscillation in an adjacent longitudinal mode is started by a minute change in a gain peak, causing conflict with the oscillation in the original oscillating mode and thus leading to noise. When multiple longitudinal modes are used, the change in the intensity of each mode is averaged and is not influenced by the disturbances. Thus, stable low-noise characteristics can be obtained.
A method for obtaining further stable self-oscillation characteristics is disclosed in Japanese Laid-Open Publication No. 63-202083. In this publication, a self-oscillation type semiconductor laser has been realized by forming a layer which can absorb output light.
Japanese Laid-Open Publication No. 6-260716 reports that the characteristics have been improved by substantially equalizing the energy gaps of an active layer and an absorption layer. In particular, the energy gaps of a strained quantum well active layer and a strained quantum well saturable absorption layer are substantially equal to each other, so as to obtain good self-oscillation characteristics. A similar configuration is described in Japanese Laid-Open Publication No. 7-22695.
However, the inventors of the present invention have found that good self-oscillation characteristics are not obtained by only substantially equalizing the energy gaps of a saturable absorption layer and an active layer.
The present invention is aimed at providing a semiconductor laser having stable self-oscillation characteristics effective for noise reduction by examining the energy gap difference between a saturable absorption layer and an active layer, as well as a method for fabricating such a semiconductor laser and an optical disk device using such a semiconductor laser.
The semiconductor laser of this invention includes an active layer having a quantum well layer and a cladding structure sandwiching the active layer, wherein the cladding structure includes a saturable absorption layer and an optical guide layer for increasing a confinement factor of the saturable absorption layer, and the energy gap of the saturable absorption layer is smaller than the energy gap between ground states of the quantum well layer of the active layer by 30 to 200 meV, whereby the above objective is attained.
Preferably, the thickness of the saturable absorption layer is in a range of about 10 to about 100 xc3x85.
A plurality of saturable absorption layers may be formed.
Preferably, the energy gap of the saturation absorption layer is smaller than the energy gap between ground states of the quantum well layer of the active layer by 50 to 100 meV.
Preferably, the optical guide layer has a band gap which is larger than a band gap of the saturable absorption layer and smaller than band gaps of the other layers of the cladding structure.
Preferably, the thickness of the optical guide layer is in a range of 300 to 1200 xc3x85.
The optical guide layer may be divided into a plurality of portions in the cladding structure.
The optical guide layer may be adjacent to the saturable absorption layer in the cladding structure.
Preferably, the saturable absorption layer is doped with impurities of 1xc3x971018 cmxe2x88x923 or more.
Preferably, the active layer has a multiple quantum well structure.
In the method for fabricating a semiconductor laser according to the present invention, the semiconductor laser includes an active layer having a quantum well layer and a cladding structure sandwiching the active layer, the cladding structure including a saturable absorption layer and an optical guide layer for increasing a confinement factor of the saturable absorption layer, the energy gap of the saturable absorption layer being smaller than the energy gap between ground states of the quantum well layer of the active layer by 30 to 200 meV, characteristics of the semiconductor laser varying with time after start of laser oscillation but being substantially fixed after a lapse of about one minute. The method includes the stabilizing step for varying the characteristics obtained immediately after the start of laser oscillation to obtain the substantially fixed characteristics, whereby the above objective is attained.
In one embodiment, the characteristics are current-light output power characteristics.
In one embodiment, the stabilizing step comprises the step of reducing a threshold current by an aging process.
In one embodiment, the stabilizing step comprises the step of reducing a threshold current by annealing.
In one embodiment, the threshold current is reduced from a value obtained immediately after the start of laser oscillation by 5 mA or more by the stabilizing step.
The optical disk device according to the present invention includes: a semiconductor laser; a converging optical system for converging a laser beam emitted from the semiconductor laser on a recording medium; and an optical detector for detecting the laser beam reflected from the recording medium, wherein the semiconductor laser includes an active layer having a quantum well layer and a cladding structure sandwiching the active layer, the cladding structure including a saturable absorption layer and an optical guide layer for increasing a confinement factor of the saturable absorption layer, and the energy gap of the saturable absorption layer is smaller than the energy gap between ground states of the quantum well layer of the active layer by 30 to 200 meV, whereby the above objective is attached.
In one embodiment, the semiconductor laser oscillates in a single mode when information is recorded on the recording medium, and operates in a self-oscillation mode when information recorded on the recording medium is reproduced.
In one embodiment, the optical detector is disposed near the semiconductor laser.
In one embodiment, the optical detector includes a plurality of photodiodes formed on a silicon substrate, and the semiconductor laser is disposed on the silicon substrate.
In one embodiment, the silicon substrate includes a concave portion formed on a principal surface thereof and a micromirror formed on a side wall of the concave portion, the semiconductor laser is disposed in the concave portion, and the angle formed between the micromirror and the principal surface is set so that the laser beam emitted from the semiconductor laser proceeds in a direction substantially vertical to the principal surface of the silicon substrate after being reflected from the micromirror.
In one embodiment, a metal film is formed on a surface of the micromirror.
In one embodiment, the active layer and the cladding structure are formed of AlxGayIn1xe2x88x92xxe2x88x92yP material (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, where x and y are not zero simultaneously).
Alternatively, the semiconductor laser of the present invention includes an active layer including a quantum well layer and a saturable absorption layer, wherein the energy gap of the saturable absorption layer is smaller than the energy gap between ground states of the quantum well layer of the active layer by 30 to 200 meV, whereby the above objective is attained.
Alternatively, the semiconductor laser of the present invention includes an active layer including a quantum well layer and a cladding structure sandwiching the active layer, wherein the cladding structure includes a saturable absorption layer, and the energy gap of the saturable absorption layer is smaller than the energy gap between ground states of the quantum well layer of the active layer by 30 to 200 meV, whereby the above objective is attained. Preferably, the thickness of the saturable absorption layer is in the range of about 10 to about 100 xc3x85.
A plurality of saturable absorption layers may be formed.
Preferably, the energy gap of the saturation absorption layer is smaller than the energy gap between ground states of the quantum well layer of the active layer by 50 to 100 meV.
Preferably, the saturable absorption layer is doped with impurities of 1xc3x971018 cmxe2x88x923 or more.
Preferably, strain is applied to the quantum well layer and the saturable absorption layer.
Preferably, the active layer has a multiple quantum well structure.